1. Field of the Invention
The present invention relates to a polarizing conversion device and a polarizing illumination device for generating, from incident light beams as randomly polarized beams, illuminating beams that have a more uniform light intensity distribution in an illumination region than that of the incident beams and are polarized in almost the same direction. Furthermore, the present invention relates to a display apparatus and a projection display apparatus using these devices.
2. Description of Related Art
A polarizing illumination device capable of efficiently generating the same type of polarized light beams is ideal as an illuminating device for use in a display apparatus, such as a liquid crystal apparatus, which employs a panel for modulating polarized light beams. Accordingly, an illuminating optical system has been proposed that converts random polarized light beams emitted from a light source into the same type of polarized light beams and illuminates a liquid crystal apparatus with the light beams so that a bright display is achieved. Japanese Unexamined Patent Publication No. 7-294906 discloses an image display apparatus equipped with such an illuminating optical system.
The principal part of the illuminating optical system disclosed in Japanese Unexamined Patent Publication No. 7-294906 will be briefly described with reference to FIG. 15. This optical system mainly comprises a lens plate 910, a plurality of polarizing beam splitters 920, a plurality of reflecting prisms 930, and a plurality of .lambda./2 phase plates 940. Incident beams as randomly polarized beams are separated into two types of polarized beams (P polarized beams and S polarized beams) through the polarizing beam splitters 920 which are respectively provided with polarizing separation planes 331 and the reflecting prisms 930 which are respectively provided with reflecting planes 332. After the separation, the polarization direction of polarized beams of one of the types is matched with that of polarized beams of the other type by using the .lambda./2 phase plates 940, thereby obtaining polarized beams of the same type and illuminating a liquid crystal device 950 with the light beams. In general, since a space for forming two types of polarized beams therein is needed in the polarized beam separation process, the optical system is inevitably widened. Accordingly, this optical system reduces the diameter of the beams, which are incident on the respective polarizing beam splitters 920, to less than about half the diameter of small lenses 911 formed in the lens plate 910 by means of the small lenses 911, and places the reflecting prisms (reflecting planes) 930 in the spaces produced by the reduction of the diameter of the beams, whereby the same type of polarized beams are obtained without widening the optical system.
The optical system disclosed in Japanese Unexamined Patent Publication No. 7-294906 has, however, the following problems.
In reducing the diameter of the beam by the lens, generally, the minimum beam diameter is almost directly and exclusively determined by the refractive power of the lens and parallelism of the light beam incident on the lens. That is, in order to reduce the beam diameter to less than half the lens diameter as in the optical system disclosed in Japanese Unexamined Patent Publication No. 7-294906, it is necessary to use a lens having an extremely high refractive power (in other words, a lens having an extremely small F-number) and a light source capable of emitting a light beam having extremely high parallelism. However, a real light source has a limited emission area. Therefore, parallelism of the light beam emitted from the light source is not always good.
On the other hand, the polarizing separation ability of the polarizing separation plane formed in the polarizing beam splitter is highly dependent on the incident angle of light. In other words, when the light that is incident on the polarizing separation plane has a large angular component, the polarizing separation plane cannot exhibit an ideal polarizing separation ability, the S polarized beam mixes into the P polarized beam transmitting through the polarizing separation plane, and the P polarized beam mixes into the S polarized beam reflected from the polarizing separation plane. Consequently, it is impossible to excessively increase the refractive power of the small lens used for reducing the diameter of the beam.
For the above reasons, it is difficult to sufficiently reduce the diameter of the light beam that is incident on the polarizing beam splitter, and, in actuality, a relatively large amount of light also directly enters the reflecting prism adjoining the polarizing beam splitter. The light that is directly incident on the reflecting prism is reflected by the reflecting plane, enters the adjoining polarizing beam splitter, and is separated into two types of polarized beams by the polarizing separation plane in the same manner as the light beam that is directly incident on the polarizing beam splitter. The light beam that is incident on the polarizing beam splitter through the reflecting prism and the light beam that is directly incident on the polarizing beam splitter are different by 90.degree. in the incident with respect to the polarizing beam splitter. As a consequence of the existence of the light beam directly incident on the reflecting prism, the S polarized beam directly incident on the reflect ng prism and separated through the polarizing beam splitter mixes into the P polarized beam that transmits through the polarizing beam splitter without changing its direction of travel. Similarly, the S polarized beam mixes into the P polarized beam that directly enters the polarizing beam splitter and is emitted through the reflecting prism and the .lambda./2 phase plate. Since the S polarized beam mixed into the P polarized beam because of the existence of the light beam directly incident on the reflecting prism is quite unnecessary for the liquid crystal device, it is absorbed by a polarizing plate and generates heat, which is the main factor that increases the temperature of the polarizing plate.
Thus, in the process in which the conventional optical system disclosed in Japanese Unexamined Patent Publication No. 7-294906 converts random light beams emitted from the light source into polarized beams of the same type, a relatively large number of polarized beams of another type inevitably mix. As a result, the polarized beams, which are unnecessary for display and are polarized in a different direction, are required to be absorbed by the polarizing plate in order to obtain an extremely bright display image. In addition, a large cooling device is essential to restrict the increase in temperature of the polarizing plate caused by the absorption of the polarizerd beams.